Passive split ring resonator for continuous physiological sensing through conductivity measurements

The Split Ring Resonator (SRR) has been developed and explored for a number of sensing technologies and devices. A SRR can be equivalently regarded as an LC circuit; changes in the dielectric environment will change the equivalent capacitance of the resonator, resulting in a shift of the resonant frequency as well as the quality factor (Q-factor).This makes the device a promising application for continuous personal health monitoring throughout the day. In this work, we are developing a passive radio frequency sensor based on ring resonator designs. The targeted frequency band is within 2.4- 2.5GHz ISM (Industrial-Scientific-Medical radio band) and is available for medical devices. The resonator structure is first simulated using Finite Difference Time Domain (FDTD) method by CST Microwave Studio to determine the resonant frequency. Then for the experimental study, a microstrip transmission line with a double split ring resonator (DSRR) was fabricated on a printed circuit board (PCB) with biocompatible PVC coating on top. Tuning the thickness and material of the biocompatible coating can further improve the biocompatibility, Q-factor, and resulting sensitivity (mS) of the device. Reflection spectrum (S11) is measured using a network analyzer at 100 mW. The current design senses changes in conductivity down to 0.5 mS. By reducing coating thickness, reducing the spacing between resonators, and with more efficient resonator designs we expect to further improve this sensitivity. This sensor could be utilized by either implanted into the interstitial layer beneath the skin or embedded into a contact lens to sense tear salinity levels.